Differences Between The 2
With Evo-Chansey you have better Defence (178,5=178) and better Sp.Def(370,5=370). Blissey has 130Def and 307Sp.Def [With the given EVs and Nature/Items Ofc].
You lose the power of Leftovers with Chansey, If you go with Blissey you get 1/16 from it that gives 44(44.625) HP regen each turn
Also Blissey has slightly more Attack/Speed/And HP
Both of them can take Ice beams a whole day and if he switch to a fighting type back to Gliscor....